white spots question

Problem

1. Noticing some white spots/patches on a few tail fins just in the last few days

2. White spots/ patches on a few fishes tail fin

3. Added some salt with last nights water change.

Tank/Water

4. Tank size and ages, numbers and sizes of fish.
110 gal, 5 months old, 12 discus only, 3-5" in size

5. Water change regime (What percentage and how often).
30-40 % daily

6. How long has tank been running? Is it bare bottom? If you have substrate, what type and how deep is it?
BB tank about 5 months

7. Do you age your water? If you do for how long and what is the ph swing.
Do not age water

8. Parameters and water source;

- temp _84-86____

- ph __7.6___

- ammonia reading _0.0___

- nitrite reading _0.0___

- nitrate reading _15-40___

What type of water or combinations of water sources do you use? If it is an RO/tap/well water mix, please list percentages in the mix.

- municipal water XX

9. Any new fish, plants or inverts added recently.
Nothing added

10. Please tell us what you feed your fish and how often. This can be critical information for solving the problem so be as specific as you can.
Beefheart mix 2x daily, flake once daily

11. Include any pictures or videos you have which shows the symptoms. If you can't add them to this post, please provide a link to them.